Wireless network including a time slot sorting mechanism

ABSTRACT

The invention relates to a wireless network comprising terminals and an assigned central station, which network, after receiving requests for the wireless transmission of packets between a transmitting and a receiving terminal during a time multiplex frame, is provided for assigning time slots of a following time multiplex frame for the wireless transmission of packets from a transmitting to a receiving terminal. After reception of all the requests the central station determines a first subset which contains all the transmitting terminals that intend to transmit packets to a plurality of receiving terminals, and a second subset of the rest which contains the transmitting terminals. The order in which the transmitting terminals of the first subset transmit is determined in accordance with the decreasing number of receiving terminals assigned to a transmitting terminal. The receiving terminals of the first subset assigned to a transmitting terminal are subdivided into a first group and into a second group which contains all the other receiving terminals, and the receiving terminals of the second group are selected first. Within the two groups the order of reception of the receiving terminals is determined in accordance with the transmission order as a transmitting terminal.

[0001] The invention relates to a wireless network comprising aplurality of terminals and an assigned central station, which network,after receiving requests for the wireless transmission of packetsbetween a transmitting and a receiving terminal during a time multiplexframe, is provided for assigning time slots of a following timemultiplex frame for the wireless transmission of packets from atransmitting to a receiving terminal.

[0002] Any data or packets can be sent via a wireless network of thistype. A packet transmission is effected, for example, in theasynchronous transfer mode (ATM=asynchronous transfer mode), which hasbeen developed for transmitting multimedia data between network nodes ordevices respectively of a network node of a network. Prior to aconnection set-up, for example between two network nodes in such an ATMnetwork agreements about transmission parameters (for example, aboutbandwidth) are transported and according to the agreement various typesof data (for example video and audio data) are inserted into cells.These cells are then transmitted over a single link to a receivingdevice. The receiving device verifies whether the received data havebeen transmitted error-free and, if necessary, the receiving devicereturns data to the transmitting device in response to the receivedcells.

[0003] The asynchronous transfer mode has actually been developed fortransmitting data through wire-bound media (for example optical cablesor copper cables). However, there are also wireless ATM networks thathave been developed as a substitute for the wire-bound media. Such awireless network transmits, for example, data over radio or infraredlinks and is known, for example, from EP 0 831 620 A2. A protocol isused here for the MAC layer (MAC=Medium Access Control) of a wirelessATM network.

[0004] The invention has for its object to provide a wireless network inwhich an optimized time slot sorting algorithm is used.

[0005] The object is achieved by a wireless network of the type definedin the opening paragraph, in that, after all the requests have beenreceived, the central station is provided for

[0006] determining a first subset which contains all the transmittingterminals that intend to transmit packets to a plurality of receivingterminals, and a second subset which contains the rest of thetransmitting terminals,

[0007] determining the order in which the transmitting terminals of thefirst subset transmit in accordance with the decreasing number ofreceiving terminals assigned to a transmitting terminal,

[0008] subdividing the receiving terminals of the first subset assignedto a transmitting terminal into a first group which contains all thereceiving terminals already used as transmitting terminals, and into asecond group which contains all the other receiving terminals,

[0009] determining the receiving order in the two groups in accordancewith the transmission order as a transmitting terminal, and

[0010] first selecting the receiving terminal of the second group.

[0011] In such a wireless network, which implements a TDMA method, acertain frequency range is used. It is then impossible for a transceiverdevice of a central station (base station or central terminal) to switchover from a reception mode to a transmission mode and vice versa withoutdelay. There is a non-negligible minimum time abbreviated to time MTbetween the two modes. The time MT is a parameter of a radio system andis to be taken into account by the part of a transceiver deviceresponsible for the Media Access Control (MAC). For taking all possibletraffic ratios in the wireless network into account and giving thevarious terminals enough time to switch-over their transmission mode, atime is defined that is longer than the time MT and is referred to astime OTT. The delay caused by the time OTT is optimized by theinvention.

[0012] The central station sorts out the transmission order of thetransmitting terminals and the receiving order of the receivingterminals assigned to a transmitting terminal. A first subset thencontains all the transmitting terminals that intend to transmit packetsto a plurality of receiving terminals, and a second subset contains therest of the transmitting terminals. The transmitting terminals of thefirst subset are sorted out so that first the transmitting terminal thathas the most receiving terminals can transmit and, finally, thetransmitting terminal that has the fewest assigned receiving terminals.The first subset of receiving terminals assigned to a transmittingterminal are subdivided into two different groups. A first groupcontains all the receiving terminals that have previously already beenassigned as transmitting terminals. The order of the two groups isdetermined so that the receiving terminals of the second group canreceive data first.

[0013] In the claims 2 and 3 are shown different methods of subdividingthe transmitting terminals of the second subset into the transmissionorder of the first subset. The invention further relates to a centralstation in a wireless network having a plurality of terminals.

[0014] Examples of embodiment of the invention will be further explainedwith reference to FIGS. 1 and 2. The Figures show each examples ofembodiment of a network for packet transmission.

[0015] The network comprising a base station configuration as shown inFIG. 1 includes various base stations 1 to 3 which control thecommunication between various wireless terminals 4. The base station 1is coupled via a connecting station 5 (gateway) to a wire-bound network6 and exchanges packets between the connecting station 5 and a certainbase station 2 and/or 3 in dependence on the address of a packet. Theconnecting station 5 is used for exchanging packets containing, forexample, audio and video data between the wire-bound and a wirelessnetwork which comprises base stations 1 and 3 and terminals 4. The basestations 2 and 3 include each a transceiver device by which theyexchange data with the terminals 4 via radio links 7. The base stations1 to 3 and the connecting station 5 are typically interconnected viaoptical or metallic cables.

[0016] The network shown in FIG. 2 with an ad hoc configuration includesvarious wireless terminals 8 to 11, of which one terminal referred to asa controller controls the communication between the terminals. Theterminal 8 is coupled to a wire-bound network 13 via a connectingstation 12 (gateway). The connecting station 12 is typically connectedto the wire-bound network and the terminal 8 via optical or metalliccables. The wireless terminals 8 to 11 (and also the terminals 4 in FIG.1), which have each a transceiver device and at least one terminalstation, exchange data via radio links 14. A terminal station may be,for example, a personal computer, a video camera, a digital telephone, adigital television or a set top box.

[0017] As discussed above, one of the terminals in FIG. 2 is arranged asa central controller, which controls the radio traffic between theterminals 8 to 11. For example, the terminal 11 can be the centralcontroller. An exchange of data may be effected between the terminals 8to 11. An exchange of control data mainly takes place between a terminal8 to 10 and the controller 11. However, it is alternatively possible forthe terminals 8 to 10 to have a direct exchange of control data witheach other.

[0018] The communication in the wireless network is based on a TDMAframe (TDMA=Time-Division Multiple Access), which includes controlchannels or control time slots and data channels or data time slots. Viaa control channel each terminal may request one or more data channelsfrom a base station (compare FIG. 1) or a controller (compare FIG. 2) totransmit cells. The base station or the controller assign 4 or 8 to 10data channels to the terminals, so that data can be transmitted duringthe following TDMA frame following a request.

[0019] In such a network as shown in FIG. 1 or 2, which implements aTDMA method, a certain frequency range is used. It is then impossiblefor a transceiver device of a base station or of a terminal to switchover without delay from a reception mode to a transmission mode and viceversa. There is a non-negligible minimum time, which is referred to forshort as time MT, between the two modes. The time MT is a parameter of aradio system and is to be taken into account by the part of atransceiver device responsible for the medium access control (MAC).

[0020] To take all possible traffic ratios in the wireless network intoaccount and give various terminals enough time to change over theirtransmission mode, a time is defined that is longer than the time MT andis referred to as change-over time OTT. For optimizing the delay in thewireless network, which delay is caused by the change-over time OTT, analgorithm to be described hereinafter is used for assigning time slotsfor the data transmission.

[0021] With the algorithm to be described, a number N of time slotsdetermined by the base station or the controller is started from, whichtime slots are rendered available by a TDMA frame for the datatransmission. The algorithm decides which N_i_j time slots are assignedfor the transmission of data from a transmitting terminal WT_i to areceiving terminal WT_j for the following TDMA frame. N_i_j thus denotesthe number of time slots for a terminal WT_I that would like to transmitpackets to the terminal WT_j. For example, 3 packets are to betransmitted from a terminal WT_1 to a terminal WT_2 {(N=3)_1_2)}, 4packets from the terminal WT_1 to the terminal WT_4 {(N=5)_1_4}, 4packets from the terminal WT_1 to the terminal WT_3 {(N=4)_1_3}, 1packet from the terminal WT_3 to the terminal WT_1 {(N=1)_3_1}, 2packets from the terminal WT_3 to the terminal WT_2 }(N=2)_3_2}, 2packets from the terminal WT_2 to the terminal WT_3 {(N=2)_2_3}, 3packets from the terminal WT_2 to the terminal WT_4 {(N−3)_2_4} and 5packets from the terminal WT_4 to the terminal WT_2 {(N=5)_4_2}. Here(N=x)_i_j means that N_i_j time slots are provided for a terminal WT_I,which transmits x packets to the terminal WT_j.

[0022] For example, the following transmission order may be determinedwithout the algorithm being applied:

[0023] |(N=3)_1_2), (N=5)_1_4, (N=4)_1_3, (N=1)_3_1, (N=2)_3_2,(N=2)_2_3, (N=3)_2_4, (N=5)_4_2|

[0024] With this transmission order there is a delay caused by thechange-over time OTT during a mode change from (N=4)_1_3 to (N=1)_3_1,because the terminal WT_1 is to change over from transmission toreception mode and the terminal WT_3 from the reception to thetransmission mode, during a mode change from (N=2)_3_2 to (N=2)_2_3,because the terminal WT_3 is to change over from transmission toreception mode and the terminal WT_2 from the reception to thetransmission mode, and during a mode change from (N=3)_2_4 to (N=5)_4_2,because the terminal WT_2 is to change over from the transmission to thereception mode and the terminal WT_4 from the reception to thetransmission mode.

[0025] The following algorithm minimizes the delay caused by thechange-over time. First a variable R(i) is defined for each terminalWT_I, which denotes the number of receiving terminals with which WT_iexchanges data in the following TDMA frame. For each N_i_j, j thenvaries from 1 to R(i) (=1, . . . , R(i)). The total number of time slotsthat are reserved for a terminal WT_i may then be indicated by thevariable S(i) with

S(i)=N _(—) i_1+N_i_2+ . . . +N _(—) i _(—) R(i)

[0026] For the example given above, the result is:

S(1)=12,S(2)=5,S(3)=3,S(4)=S and R(1)=3,R(2)=2,R(3)=2,R(4)=1.

[0027] Subsequently, all the transmitting terminals WT_i are dividedinto two subsets A and B. The subset A contains all the transmittingterminals WT_i with R(i)>1 and the subset B all the transmittingterminals WT_i with R(i)=1.

[0028] With the example given above the terminals WT_1 with R(1)=3, WT_2with R(2)=2 and WT_3 with R(3)=2 form part of the subset A and theterminal WT_4 with R(4)=1 forms part of subset B.

[0029] For the subset A the sorting order is determined so that theterminal WT_i transmits its S(i) packets earlier than terminal WT_j ifR(i)>R(j). If R(i)=R(j), the transmission order is selected at random.After the sorting order has been determined in this way, the order ofthe receiving terminal WT_is determined for a transmitting terminalWT_i. The receiving terminals WT_j receiving packets from a transmittingterminal WT_i are divided into two groups A_1 and A_2.

[0030] The group A_1 comprises all the receiving terminals WT_j thathave already terminated their transmission to other terminals (beforeWT_i starts transmitting packets). The group A_2 comprises all thereceiving terminals WT_j that have not yet started their transmissionand start the transmission after the terminals WT_i have terminatedtheir transmission.

[0031] The order of the receiving terminals WT_i for a transmittingterminal WT_i is then fixed so that all the terminals WT_j of the groupA_1 receive the packets from WT_i later than those of the group A_2.Within the groups A_1 and A_2 the terminals WT_j are sorted as follows:

[0032] The group A_1 (first group) is sorted so that the transmittingterminal WT_i transmits its packets in N_i_m time slots to the receivingterminal WT_m earlier than its packets in N_i_n time slots to thereceiving terminal WT_n, if the terminal WT_m terminates itstransmission earlier than the terminal WT_n (WT_m is a transmittingterminal earlier than WT_n).

[0033] The group A_2 (second group) is sorted so that the transmittingterminal WT_i transmits its packets in N_i_m time slots to the receivingterminal WT_m earlier than its packets in N_i_n time slots to thereceiving terminal WT_n, if the terminal WT_m starts its transmissionearlier than the terminal WT_n (WT_m is a transmitting terminal earlierthan WT_n).

[0034] This terminates the sorting algorithm for the subset A.

[0035] In the example given above, when the sorting algorithm isimplemented for the subset A, first a sorting is made for thetransmitting terminal WT_1, then for the transmitting terminal WT_3 andthen for the transmitting terminal WT_2. After WT_1 could first becarried out a sorting for the transmitting terminal WT_2 and then forthe transmitting terminal WT_3, because R(2)=R(3). The algorithmdescribed above for the groups A_1 and A_2 then performs a sorting ofthe receiving terminals receiving packets from WT_1. There is then thefollowing order:|(N=4)_1_3, (N=3)_1_2, (N=5)_1_4|

[0036] For the transmitting terminals WT_3 and WT_2 is then used thealgorithm for the groups A_1 and A_2. All in all there is then thefollowing sorting order for the subset A::

[0037] |(N=4)_1_3, (N=3)_1_2, (N=5)_1_4, (N=2)_3_2, (N=1)_3_1,(N=3)_2_4, (N=2)_2_3|

[0038] After the sorting for the subset A has ended, the following stepsare made for the sorting of the terminals of the subset B. The subset Bcomprises K terminals WT_i. A transmit pointer p is defined, whichfeatures the terminal WT_x transmitting last, before a terminal of thesubset B starts its transmission, and a receive pointer q which featuresthe terminal receiving data last from an arbitrary terminal. Fordetermining the first transmitting terminal from the subset B, p and qrespectively point to the last transmitting or receiving terminal of thesorting order for the subset A determined so far.

[0039] In a first step a test is made whether K=1. If this is the case,only a single terminal WT_i of the subset B is put in the sorting orderas the last transmitting terminal. This also determines the associatedreceiving terminal of the transmitting terminal, because all thetransmitting terminals in the subset B have only one receiving terminal.The sorting is then ended for the subset B.

[0040] In a second step a test is made whether K>1. If this is the case,the terminal WT_i is removed from the subset B and marked as a selectedterminal WT_i if there is a terminal WT_i that is not featured by thereceive pointer q and is not featured by the last transmit pointer p bythe receiving terminal receiving data from the terminal WT_i. If thiscondition is satisfied, the terminal WT_i is removed from the subset Band a third step is proceeded to (terminal WT_i has not previouslyreceived any data and the associated receiving terminal has notpreviously transmitted any data). If this condition is not satisfied, anarbitrary terminal WT_i is selected from the subset B, which terminal isnot featured by the last receive pointer q and is marked as a selectedterminal WT_i (terminal WT_i has not previously received any data, butits associated receiving terminal has previously transmitted data). Thenthe third step is proceeded to.

[0041] In the third step the terminal WT_i selected in the second stepis put at the end of the assignment list. The selected terminal WT_i isfeatured by the transmit pointer p and the terminal receiving data fromWT_i is featured by the receive pointer q. K is decremented by 1 and thefirst step is proceeded to.

[0042] With the example there is only the terminal WT_4, which belongsto the subset B. This terminal WT_4 would like to transmit five packetsto receiving terminal 2. In this way there is the following optimizedsorting order for the two subsets A and B:

[0043] |(N=4)_1_3, (N=3)_1_2, (N=5)_1_4, (N=2)_3_2, (N=1)_3_1,(N=3)_2_4, (N=2)_2_3, (N=5)_4_2|

[0044] In this example there is only one delay as a result of achange-over period of the terminal WT_2, because this terminal, aftertransmitting two packets to the terminal WT_3, changes to the receptionmode to receive five packets from the terminal WT_4. In the otherterminal there is no delay resulting from the change-over period. Forexample, the terminal WT_1, after transmitting four packets to theterminal WT_3, after transmitting three packets to the terminal WT_2 andafter transmitting five packets to the terminal WT_4 during the periodof two time slots, has enough time to change from the transmission tothe reception mode. During this period of two time slots the terminalWT_3 transmits two packets to the terminal WT_2.

[0045] It is alternatively possible to determine the sorting order sothat the terminals of the second subset transmit or receive respectivelyearlier than the terminals of the first subset. The terminals of thesecond subset, after the sorting order of the first subset has beendetermined, can also be distributed over the sorted terminals of thefirst subset. It should then be borne in mind that a transmittingterminal is a receiving terminal neither in the previous nor in thesubsequent time slot and a receiving terminal is a transmitting terminalneither in the preceding nor in the subsequent time slot.

1. A wireless network comprising a plurality of terminals and anassigned central station, which network, after receiving requests forthe wireless transmission of packets between a transmitting and areceiving terminal during a time multiplex frame is provided forassigning time slots of a following time multiplex frame for thewireless transmission of packets from a transmitting to a receivingterminal, characterized in that after reception of all the requests thecentral station is provided for determining a first subset of all thetransmitting terminals that intend to transmit packets to a plurality ofreceiving terminals, and a second subset containing the rest of thetransmitting terminals, determining the order in which the transmittingterminals of the first subset transmit in accordance with the decreasingnumber of receiving terminals assigned to a transmitting terminal,subdividing the receiving terminals of the first subset assigned to atransmitting terminal into a first group which contains all thereceiving terminals already used as transmitting terminals, and into asecond group which contains all the other receiving terminals,determining the receiving order in the two groups in accordance with thetransmission order as a transmitting terminal and first selecting thereceiving terminal of the second group.
 2. A wireless network as claimedin claim 1, characterized in that the central station is provided fordetermining the transmission order of the transmitting terminals of thesecond subset in such a way that first all the transmitting terminalsare selected that have not previously been either a transmitting or areceiving terminal and then all the transmitting terminals are selectedthat have not previously been a receiving terminal and in that thetransmitting terminals of the second subset are provided fortransmitting either before or after the transmitting terminals of thefirst subset.
 3. A wireless network as claimed in claim 1, characterizedin that the central station is provided for dividing the transmittingterminals of the second subset into the transmission order of the firstsubset in such a way that a transmitting terminal is not a receivingterminal in the preceding and following time slot and a receivingterminal is not a transmitting terminal in the preceding and followingtime slot.
 4. A central station in a wireless network comprising aplurality of terminals, which central station, after receiving requestsfor the wireless transmission of packets between a transmitting and areceiving terminal during a time multiplex frame is provided forassigning time slots of a following time multiplex frame for thewireless transmission of packets from a transmitting to a receivingterminal, characterized in that after reception of all the requests thecentral station is provided for determining a first subset of all thetransmitting terminals that intend to transmit packets to a plurality ofreceiving terminals, and a second subset containing the rest of thetransmitting terminals, determining the order in which the transmittingterminals of the first subset transmit in accordance with the decreasingnumber of receiving terminals assigned to a transmitting terminal,subdividing the receiving terminals of the first subset assigned to atransmitting terminal into a first group which contains all thereceiving terminals already used as transmitting terminals, and into asecond group which contains all the other receiving terminals,determining the receiving order in the two groups in accordance with thetransmission order as a transmitting terminal and first selecting thereceiving terminal of the second group.